To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and the guns fired to create openings in a casing and to extend perforations into the surrounding formation.
The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix can form around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnel, may be dictated by a variety of factors including formation properties, explosive charge properties, pressure conditions, fluid properties, and so forth. The shock damaged region and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells.
One method of obtaining clean perforations is underbalanced perforating, referred by Schlumberger proprietarily as “PURE”. The perforating process results in a wellbore pressure which drops rapidly to a value below the formation pressure. This dynamic, or transient underbalance, cleans the perforation damage, thereby improving well performance.
There is a continuing need to improve that process to optimize fluid communication with reservoirs in formations of a well. The present application describes a number of embodiments addressing a number of issues associated therewith.